This invention relates to compositions of matter comprising polydentate ligand complexes of chromium, vanadium and titanium having an electrochemically generated 2+ formal valence charge on the metal atom in the complex, processes for their formation, processes for their use as reducing agents in bleaching or brightening lignocellulosic pulps, principally wood pulp, either as mechanical wood pulp, or chemical wood pulp and to the brightened pulps produced thereby.
Reductive bleaching or brightening of lignocellulosic pulps particularly mechanical wood pulps has long been practiced. The principal reagent for this purpose has been hydrosulfite (dithionite) ion (S.sub.2 O.sub.4.sup.=) exployed principally as its zinc or sodium salts in aqueous solution. More recently sodium borohydride (NaBH.sub.4) in various formulations has also been employed as a replacement for hydrosulfite.
The problems and limitations involved in the use of both reagents, such as, lack of stability in solution or in the presence of oxygen, the inability to reuse spent reagent requiring its environmentally acceptable disposal and the limited ability of either reagent to produce high brightness pulp are well known to those skilled in the art.
Applicants have discovered a new series of reducing agents which are capable of eliminating or minimizing most of the deficiencies of borohydride and hydrosulfite in reductive bleaching of lignocellulosic pulp.
The use of reducing agents generated electrochemically, particularly of reducing agents generated electrochemically in situ in the process, to reductively bleach lignocellulosic pulp does not appear to have been suggested in the prior art.